A major breakthrough in FCC catalysts came in the early 1960's, with the introduction of molecular sieves or zeolites. These materials were incorporated into a matrix of amorphous and/or amorphous/kaolin materials constituting the FCC catalysts of that time. These new zeolitic catalysts, containing a crystalline aluminosilicate zeolite in an amorphous or amorphous/kaolin matrix of silica, alumina, silica-alumina, kaolin, clay or the like were at least 1,000-10,000 times more active for cracking hydrocarbons than the earlier amorphous or amorphous/kaolin containing silica-alumina catalysts. This introduction of zeolitic cracking catalysts revolutionized the fluid catalytic cracking process. New innovations were developed to handle these high activities, such as riser cracking, shortened contact times, new regeneration processes, new improved zeolitic catalyst developments, and the like.
After the introduction of zeolitic containing catalysts the petroleum industry began to suffer from a lack of crude availability as to quantity and quality accompanied by increasing demand for gasoline with increasing octane values. The world crude supply picture changed dramatically in the late 1960's and early 1970's. From a surplus of light, sweet crudes the supply situation changed to a tighter supply with an ever increasing amount of heavier crudes with higher sulfur and nitrogen contents. These heavier and higher sulfur-nitrogen crudes presented processing problems to the petroleum refiner in that these heavier crudes invariably also contained much higher metals and Conradson carbon values, with accompanying significantly increased asphaltic content.
The effects of metal contaminants and Conradson carbon on a zeolite containing FCC catalyst has been described in the literature as to their highly unfavorable effect in lowering catalyst activity and selectivity for producing liquid fuel products comprising gasoline production and their equally harmful effect on catalyst life.
The heavier crude oils also contained more of the heavier compounds comprising asphaltenes and polycyclic compounds that yield less or a lower volume of a high quality FCC gas oil charge stock which normally boil below about 1025.degree. F., and are usually processed, so as to contain total metal levels below 1 ppm, preferably below 0.1 ppm, and Conradson carbon values substantially below 1.0.
However, with an increased supply of the heavier, less desirable crudes, which provide lowered yields of gasoline, and the increasing demand for liquid transportation fuels, the petroleum industry must search for and provide processing schemes to utilize these heavier crudes in producing gasoline and other needed liquid fuel products. Many of these processing schemes have been described in the literature. These include Gulf's Gulfining and Union Oil's Unifining processes for treating residium, UOP's Aurabon process, Hydrocarbon Research's H-Oil process, Exxon's Flexicoking process to produce thermal gasoline and coke, H-Oil's Dynacracking and Phillip's Heavy Oil Cracking (HOC) processes. These processes utilize thermal cracking or hydrotreating followed by FCC or hydrocracking operations to handle the higher content of metal contaminants (Ni-V-Fe-Cu-Na) and high Conradson carbon values of 5-15. Some of the drawbacks of these types of processing are as follows: coking yields thermally cracked gasoline which has a much lower octane value than cat cracked gasoline and is unstable due to the production of gum from diolefins and requires further hydrotreating and reforming to produce a higher octane product; gas oil quality is degraded due to thermal reactions which produce a product containing refractory polynuclear aromatics and high Conradson carbon levels which are highly unsuitable for catalytic cracking; and hydrotreating requires expensive high pressure hydrogen, multi-reactor systems made of special alloys, costly operations, and a separate costly facility for the production of hydrogen.
To better understand the reasons why the industry has progressed along today's processing schemes, one must understand the known and established effects of contaminant metals (Ni-V-Fe-Cu-Na) and Conradson carbon on a zeolite containing cracking catalyst and the operating parameters of a catalytic cracking operation. Metal content and Conradson carbon are two very effective restraints on the operation of a FCC unit and may even impose undesirable restraints on a Reduced Crude Conversion (RCC) unit from the standpoint of obtaining satisfactory conversion, selectivity and catalyst life. Even relatively low levels of these contaminants are highly detrimental to the present day FCC units relying upon zeolite cracking catalysts. As metals and Conradson carbon levels are increased the operating capacity and efficiency of a reduced crude cracking process is also adversely affected or even made uneconomical. These adverse effects occur even though there is enough hydrogen in the feed to produce an ideal gasoline consisting of only toluene and isomeric pentenes (assuming a catalyst with such ideal selectivity could be devised).
The effect of increased Conradson carbon producing components in a cracking feed is to increase that portion of the feedstock normally converted to coke deposited on the catalyst. In typical gas oil operations comprising vacuum gas oils and employing a zeolite containing cracking catalyst in a fluid catalyst cracking unit, the amount of coke deposited on the catalyst averages around about 4-5 wt% of the feed. This coke production has been attributed to four different coking mechanisms, namely, contaminant coke from adverse reactions caused by metal deposits, catalytic coke caused by acid site cracking, entrained hydrocarbons resulting from pore structure adsorption and/or poor stripping, and Conradson carbon resulting from pyrolytic distillation of heavy, high molecular weight hydrocarbons, in the conversion zone. There has also been postulated two other sources of coke from reduced crudes in addition to the four above identified. They are: (1) adsorbed and absorbed high boiling hydrocarbons which do not boil or vaporize at a temperature below 1025.degree. F. and cannot be removed from catalyst particle by the present stripping operations, and (2) high molecular weight nitrogen containing hydrocarbon compounds adsorbed on the catalyst's acid sites. Both of these two new types of coke producing phenomena add greatly to the complexity of residual oil, reduced crude and resid processing. Therefore, in the processing of these high boiling crude oil fractions, e.g., reduced crudes, residual fractions, topped crude, and the like, the coke production based on feed is a summation of the four types present in gas oil processing plus coke obtained from the higher boiling unstrippable hydrocarbons and coke associated with the high boiling nitrogen containing molecules which are adsorbed on the catalyst. Coke production on clean catalyst, when processing reduced crudes, may be roughly estimated as approximately 4 wt% of the feed plus the Conradson carbon value of the heavy feedstock.
The catalyst comprising hydrocarbonaceous deposits of hydrocarbon conversion is brought back to equilibrium activity by burning off the deactivating hydrocarbonaceous material and residual coke in a regeneration zone in the presence of air thereby heating the catalyst to an elevated temperature. The regenerated catalyst at an elevated temperature is recycled back to the reaction zone. The heat generated during burning in the regeneration zone is removed in part by the heated catalyst and carried to the reaction zone for vaporization of the feed and to provide heat for the endothermic cracking reaction. Hot regeneration flue gases also remove a portion of the regeneration heat. The temperature in the regenerator is normally limited below 1600.degree. F. because of metallurgical limitations and the hydrothermal stability of the catalyst.
The hydrothermal stability of a zeolite containing catalyst is determined by the temperature and steam partial pressure at which the zeolite begins to rapidly lose its crystalline structure to yield a lower activity material considered amorphous. The presence of steam is highly critical and is generated by the burning of adsorbed and absorbed (sorbed) carbonaceous material which has a significant hydrogen content (hydrogen to carbon atomic ratios generally greater than about 0.5). This hydrocarbonaceous material deposit is obtained in substantial measure from the high boiling sorbed hydrocarbons and particularly from asphaltic or polycyclic high molecular weight materials which do not vaporize at temperatures below 1025.degree. F. These materials have a modest hydrogen content and include high boiling nitrogen containing hydrocarbons, as well as related high molecular weight porphyrins and asphaltenes. The high molecular weight nitrogen compounds usually do not boil or vaporize below about 1025.degree. F. and may be either basic or acidic in nature. The basic nitrogen compounds tend to neutralize acid cracking sites while those that are more acidic may be attracted to metal sites on the catalyst. The porphyrins and asphaltenes which also do not vaporize at temperatures up to 1025.degree. F., may contain elements other than carbon and hydrogen. As used in this specification, the term "heavy hydrocarbons" includes all carbon and hydrogen containing resid compounds that do not boil or vaporize at a temperature in the range of 650.degree. F. up to 1025.degree. F., regardless of whether other elements are also present in the compound.
The heavy metals in the feed are generally present as porphyrins and/or asphaltenes. However, certain of these metals, particularly iron and copper, may be present as a free metal or as inorganic compounds resulting from either corrosion of process equipment or contaminants from other refining processes.
As the Conradson carbon value of the feedstock increases, coke production increases and this increased load will raise the regeneration temperature; thus any given cracking-regeneration unit may be limited as to the amount of feed that can be processed, because of its Conradson carbon content.
The metal containing fractions of reduced crudes contain Ni-V-Fe-Cu in the form of porphyrins and asphaltenes. These metal containing hydrocarbons are deposited on the catalyst during processing and are cracked in the riser to deposit the metal with hydrocarbonaceous material on the catalyst. These deposits are carried by the catalyst substantially as metallo-porphyrin or asphaltene to a regeneration operation and converted to the metal oxide during regeneration. The adverse effects of the deposited metals during hydrocarbon conversion as taught in the literature are to cause non-selective or degradative cracking and dehydrogenation to produce increased amounts of deposited carbonaceous material and light gases products such as hydrogen, methane and ethane. These reaction mechanisms adversely affect the cracking selectivity, resulting in poor product yields and poor quality gasoline and light cycle oil. The increased production of light gases, while impairing the yield and selectivity of the processes, also puts an increased demand on the downstream gas plant gas compressor capacity. The increase in coke production, in addition to its negative impact on yield, also adversely affects catalyst activity-selectivity, greatly increases regenerator air demand and compressor capacity and contributes to high regenerator temperature.
Certain crudes such as Mexican Mayan or Venezuelan crudes contain abnormally high metal and Conradson carbon values. If these poor grades of crude are processed as is in a reduced crude process, they will lead to an uneconomical operation because of the high coke burning load on the regenerator and the high catalyst addition rate required to maintain catalyst activity and selectivity. The addition rate can be as high as 4-8 lbs./bbl. which at today's catalyst prices, can add as much as $2-8/bbl. as additional catalyst cost to the processing economics. It is therefore desirable to develop an economical means of processing poor grade crude oils, such as the Mexican Mayan, because of their availability and cheapness as compared to Middle East crudes.
The literature suggests many processes for the reduction of metals content and Conradson carbon values of residual oil, topped or reduced crudes and other contaminated high boiling oil fractions. One such process is that described in U.S. Pat. No. 4,243,514 and German Pat. No. 29 04 230 assigned to Engelhard Minerals and Chemicals, Inc., which patent disclosures are incorporated herein by reference thereto. These prior art processes involve contacting a reduced crude fraction or other contaminated oil fraction with a sorbent material at elevated temperatures in a sorbing zone, such as in a fluid bed contact zone to produce a product of reduced metal and Conradson carbon value. One of the sorbents described in U.S. Pat. No. 4,243,514 is an inert solid initially composed of kaolin, which has been spray dried to yield microspherical particles having a surface area below 100 m.sup.2 /g and a catalytic cracking micro-activity (MAT) value of less than 20 which is calcined at high temperature so as to achieve better attrition resistance.